regulator: add verbose error messages for invalid voltage requests

Add error messages into the regulator_check_voltage() and
regulator_check_consumers() functions which explain exactly what
is not correct about given voltage requests.  This makes
debugging regulator_set_voltage() errors easier.

Change-Id: I8b3ec8d6a78c94b436b57bd2228b8bd5c362cecd
Signed-off-by: David Collins <collinsd@codeaurora.org>